The patent application of G. Reafler, Ser. No. 116,426 filed Nov. 3, 1987 describes a new paint-coated, thermoformable sheet material of exceptional high quality. It is useful for applying protective and decorative surfaces to substrates such as automobile panels, household appliances and the like. The new paint-coated sheet materials can afford savings in paints as compared with conventional spray painting processes. Moreover, air pollution from evaporating paint solvents can be reduced.
Through the use of laminar flow coating techniques as described in the Reafler patent application, sheet materials are formed which have outstanding surface properties such as gloss and distinctness of image. A preferred species of the new protective and decorative sheet material is formed by coating an aqueous polyurethane paint on a stretchable, thermoplastic polyester carrier film. To improve the adhesion of the aqueous polyurethane paint to the polyester carrier films, an adhesion promoting coating or, as it is also called, a tie layer, is coated on the carrier film before the paint is applied.
Although previous tie layers have improved adhesion, very strong adhesion is required of sheet materials to be stretched and bonded by thermoforming to three-dimensional substrates for use as exterior coverings for automotive panels and other articles. It is difficult to achieve strong adhesion of a water-based polyurethane paint composition to a stretchable, hydrophobic polyester carrier film, especially when the paint contains light-reflective metallic flakes, even though a tie coat is used. The exposure of such materials to heat and humidity as occurs, e.g., in the normal uses of automobiles, and the stretching of the sheet materials, as occurs in thermoforming, further aggravate the problem. Hence, a sheet material with improved adhesion of the aqueous polyurethane paint, including metallic paints, to the polyester carrier film has been needed.